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Reduction of the quadruply-bridged

2 the essentially isotropic interaction with A1 is the result of direct overlap possible between its 3 s orbital and the semifilled orbital of the a carbon. The sign of the A1 coupling elements given in eq 4 must therefore be positive. The z axis along which the smallest A1 coupling constant is measured must then be perpendicular to the molecular plane. The smallest -proton coupling constant was also observed along the z axis. It follows that the -proton coupling constant of the Al-acetylene adduct is positive and its C=Ca-Ha section is also bent.As stated earlier the photoinduced spectral change observed with the Al/CjHj/Ar system can be best understood in terms of the cis-trans isomerization of the vinyl form. We therefore examined the radical 2 by the EHT (extended Huckel type) molecular orbital theory. In particular, based on the structural parameters depicted below, the total energy of the radical and the isotropic A1 coupling constant were examined as a function of , the angle of bend of the C=Ca-Ha section. The results 1 08 l H ------1-_ are shown in Figure 13. The EHT-MO theory thus states that the trans configuration at ^-40°is more stable than the linear structure ( = 0°) by ~4 kcal. The energy vs. curve does not show a minimum in the > 0 region; its shape, however, strongly suggests the possibility of a meta-stable cis configuration at =* 40°. It thus appears that the addition reaction of A1 to acetylene is symmetry restricted to yield only the cis form, and a barrier exists to prevent the conversion to the more stable trans isomer.The barrier need not be much higher than kT of the colliding atom/molecular beams since the conversion would be a unimolecular process at extremely low pressure (<10"1 2345 67torr), and the adduct remains at ambient temperature for only ~10"5 s. Most interestingly, the EHT calculation (Figure 13) also showed that the cis-to-trans conversion should lead to a decrease of ~20 G in the isotropic A1 coupling constant as observed experimentally.Acknowledgment. The author wishes to express his sincere appreciation to D. McLeod, Jr. (Union Carbide Corporation, Tarrytown, NY), for his assistance in obtaining many of the experimental results presented in this paper.

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